As I've bought two of these (good price) and have one earmarked for a Swedish SK 16, one A and one B, A = Yellow, B = Green, both '13' (of course), what I don't know, is what to make of the third one....
Can it be done as prewar, warbird, postwar....been thinking French, German, RAF, US.....suggestions?

As you can see, it's a Mk 4, or IV as some like to call them, with the long, curved rear canopy....

Depends on what's in the kit old boy. For example, the Harvard MkI had a different tail assembly and rear fuselage, earlier airframes had different undercarriage, later versions had various canopy changes and so on.
I'll e-mail the 'Squadron Signal' book to you - might give you some ideas.

Email received old boy, much obliged!
For what I understand BC-1A, AT-6A, Mk. IIB and AT-16 are all the same aircraft....just the usual paper thingy.
Since you can, for what I've read online, this Modelcraft Mk. 4, can also be built as a Mk. IIB, in other words, a Mk. IIB, BC-1A,
AT-6A and AT-16, correct?

Probably, but the box art appears to show a AT-6 Texan. The Harvard MkIV had the framed canopy, with the longer rear section, and the extended exhaust, like the Boscombe Down version I built a while back. I had to make the canopy frames, and 'fudge' the extreme rear, fixed, canopy section, as I couldn't be Rsed moulding a new one!
The kit used was Italeri, which I think is the same moulds as yours.

Yes old chap, it's the same kit.
Note that the open canopies are incorrect on that last model in the review. The rear cockpit canopy slid forward, under the central section, and the front cockpit canopy slid rearwards, over the central section. It's not fully possible to achieve this with the rear canopy in the kit, unless some surgery is undertaken.
The extra frames of the MkIV (and earler Harvard types) can easily be added using painted strips and angles of 'Sellotape' (clear tape, like 'Scotch' tape).